A magnetic memory, or a magnetic random access memory (MRAM) is a nonvolatile memory which can operate at a high speed and is rewritable for infinite times. The magnetic memory partly has gone into commercial use and is under development for the purpose of enhancing the general versatility thereof. In an MRAM, a magnetic substance is used as a storage element, and information is stored in correspondence with the direction of magnetization of the magnetic substance. Several methods have been proposed as methods of switching magnetization of the magnetic substance, but usage of a current is common to all of them. In commercializing an MRAM, how much the writing current thereof can be decreased is very important. According to Non Patent Literature 1, decrease in writing current to 0.5 mA or less, more preferably to 0.2 mA or less is required (Non Patent Literature 1). The reason is that, if the writing current is decreased to about 0.2 mA, a minimum layout is possible in a two transistors-one magnetic tunnel junction (2T-1MTJ) circuit structure proposed in Non Patent Literature 1, and cost performance which is equivalent to or higher than that of an existing volatile memory such as a DRAM or an SRAM can be realized.
Among methods of writing information to an MRAM, the most popular one is a method in which wiring for writing is provided on the periphery of a magnetic storage element and, by a magnetic field generated by a passing current through the wiring, the direction of magnetization of the magnetic storage element is switched. In this method, magnetization reversal occurs due to the magnetic field, and thus, theoretically, writing in one nanosecond or less is possible, which is suitable for realizing a high-speed MRAM. However, a magnetic field for switching magnetization of a magnetic substance whose thermostability and resistance to a disturbance magnetic field are secured is ordinarily about several tens of oersteds (10 Oe=795.77 A/m). In order to generate such a magnetic field, a current of about several milliamperes is necessary. In this case, increase in chip area cannot be avoided, and further, power consumption necessary for writing also increases, and thus, the competitiveness is inferior to those of other random access memories. In addition to this, when the element becomes finer, the writing current thereof further increases, which is not preferred also from the viewpoint of scaling.
In recent years, as means for solving such a problem, the following two methods are proposed. One is spin injection magnetization reversal. This is a method in which, in a laminated film including a first magnetic layer (magnetization free layer) having reversible magnetization and a second magnetic layer (reference layer) which is electrically connected thereto and has fixed magnetization, magnetization of the first magnetic layer (magnetization free layer) is reversed using the interaction between a conduction electron which is spin polarized when a current is passed between the second magnetic layer (reference layer) and the first magnetic layer (magnetization free layer) and a localized electron in the first magnetic layer (magnetization free layer). In reading, the magnetoresistance which develops between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer) is used. Therefore, an MRAM which uses the spin injection magnetization reversal method is a two-terminal element. Spin injection magnetization reversal occurs when the current density is equal to or higher than a predetermined level, and thus, when the size of the element becomes smaller, a current which is necessary in writing decreases. In other words, it can be said that the spin injection magnetization reversal method is excellent in scaling. However, generally, a non-magnetic layer is provided between the first magnetic layer (magnetization free layer) and the second magnetic layer (reference layer), and, in writing, a relatively large current has to be passed through the non-magnetic layer, and the resistance to rewriting and the reliability are the problems. Further, the current path for writing and the current path for reading are the same, and thus, there is also concern about miswriting in reading. As described above, spin injection magnetization reversal is excellent in scaling, but there are some barriers to the practical application thereof.
On the other hand, a magnetization reversal method which uses a current-induced domain wall motion phenomenon, which is the second method, can solve the above-mentioned problems of spin injection magnetization reversal. An MRAM which uses the current-induced domain wall motion phenomenon is disclosed in, for example, Patent Literature 1 (Patent Literature 1). In an MRAM which uses the current-induced domain wall motion phenomenon, generally, both end portions of a first magnetic layer (magnetization free layer) having reversible magnetization have magnetization fixed in directions antiparallel to each other. In such a magnetization arrangement, a domain wall is introduced into the first magnetic layer. In this case, as is reported in Non Patent Literature 2, when a current is passed in a direction of piercing the domain wall, the domain wall moves in the direction of conduction electrons, and thus, writing is possible by passing a current through the first magnetic layer (magnetization free layer) (Non Patent Literature 2). In reading information, magnetic tunnel junction (MTJ) that is provided in a region in which the domain wall moves is used to carry out reading using magnetoresistance. Therefore, an MRAM which uses the current-induced domain wall motion method is a three-terminal element, and conforms to the 2T-1MTJ structure proposed in Non Patent Literature 1 described above. Current-induced domain wall motion also occurs when the current density is equal to or higher than a predetermined level, and thus, it can be said that current-induced domain wall motion has, similarly to spin injection magnetization reversal, a scaling property. In addition to this, in an MRAM element which uses current-induced domain wall motion, a writing current does not flow through a non-magnetic layer in the magnetic tunnel junction, and further, the current path for writing and the current path for reading are different from each other. Therefore, the above-mentioned problem of spin injection magnetization reversal is solved.
Further, in Non Patent Literature 2, the current density necessary for current-induced domain wall motion is about 1×1018 [A/cm2]. In this case, when, for example, the width and the thickness of a layer in which domain wall motion occurs (magnetization free layer) are 100 nm and 10 nm, respectively, the writing current is 1 mA. This cannot satisfy the above-mentioned condition with regard to the writing current. On the other hand, as described in Non Patent Literature 3, it has been reported that, by using a material having perpendicular magnetic anisotropy as a ferromagnetic layer in which current-induced domain wall motion occurs (magnetization free layer), the writing current can be sufficiently reduced (Non Patent Literature 3). From this, it can be said that, when an MRAM is manufactured using current-induced domain wall motion, it is preferred to use a ferromagnetic substance having perpendicular magnetic anisotropy as the layer in which domain wall motion occurs (magnetization free layer).
Structures of an MRAM in which a ferromagnetic substance having perpendicular magnetic anisotropy is used as the magnetization free layer are disclosed in Patent Literature 2 and Patent Literature 3 (Patent Literatures 2 and 3).